Risk of reinfection after seroconversion to SARS-CoV-2: A population-based propensity-score matched cohort study

Importance

Serological assays detecting specific IgG antibodies generated against the Spike protein following Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection are being widely deployed in research studies and clinical practice. However, the duration and the effectiveness of the protection conferred by the immune response against future infection remains to be assessed in a large population.

Objective

To estimate the incidence of newly acquired SARS-CoV-2 infections in seropositive individuals from a population-based sample as compared to seronegative controls.

Design

Retrospective longitudinal propensity-score matched cohort study.

Setting

A seroprevalence survey including a population-based representative sample of the population from the canton of Geneva (Switzerland) was conducted between April and June 2020, immediately after the first pandemic wave. Each individual included in the seroprevalence survey was linked to a state centralized registry compiling virologically confirmed SARS-CoV-2 infections since the beginning of the pandemic.

Participants

Participants aged twelve years old and over, who developed anti-spike IgG antibodies were matched one-to-two to seronegative controls, using a propensity-score including age, gender, immunodeficiency, body mass index, smoking status and education level.

Exposure

SARS-CoV-2 seropositivity.

Main outcomes and measures

Our primary outcome was virologically confirmed SARS-CoV-2 infections which occurred from serological status assessment in April-June 2020 to the end of the second pandemic wave (January 2021). Additionally, incidence of infections, rate of testing and proportion of positive tests were analysed.

Results

Among 8344 serosurvey participants, 498 seropositive individuals were selected and matched with 996 seronegative controls. After a mean follow-up of 35.6 (Standard Deviation, SD: 3.2) weeks, 7 out of 498 (1.4%) seropositive subjects had a positive SARS-CoV-2 test, of which 5 (1.0%) were considered as reinfections. By contrast, infection rate was significantly higher in seronegative individuals (15.5%, 154/996) during a similar mean follow-up of 34.7 (SD 3.2) weeks, corresponding to a 94% (95%CI 86% to 98%, P <0.001) reduction in the hazard of having a positive SARS-CoV-2 test for seropositive subjects.

Conclusions and relevance

Seroconversion after SARS-CoV-2 infection confers protection to successive viral contamination lasting at least 8 months. These findings could help global health authorities establishing priority for vaccine allocation.

Key points

Question

Do SARS-CoV-2 antibodies confer protection against future infection?

Findings

In this retrospective matched cohort study nested in a representative sample of the general population of Geneva, Switzerland, we observed a 94% reduction in the hazard of being infected among participants with antibodies against SARS-CoV-2, when compared to seronegative controls, >8 months after initial serology assessment.

Meaning

Seroconversion to SARS-CoV-2 is associated with a large and sustained protection against reinfection.